Gamma switching amplifier

Abstract

Electronic devices with displays are configured to provide a gamma correction signal to each source driver chip driving the display. The gamma correction signal is supplied by a gamma application circuit coupled to each source driver chip. The gamma application circuit includes a switching amplifier configured to output a switching waveform and a filter to input the switching waveform and output the gamma correction signal to an input of each source driver chip. The switching amplifier functions as a switching power supply having improved power efficiency compared to conventional gamma application circuits.

I claim: 
     
       1. A liquid crystal display with gamma application circuit comprising:
 a liquid crystal display having one or more source driver chips, wherein each source driver chip includes an input stage having a digital-to-analog converter (DAC) followed by a buffering amplifier; and 
 a gamma application circuit coupled to the input stage of the one or more source driver chips with a discrete inductor to supply each of the one or more source driver chips with a gamma correction signal; 
 wherein the gamma application circuit comprises a capacitor coupled to the inductor, a switching Class D switching amplifier coupled to an analog voltage power supply and having a power efficiency equal to or greater than 80%, the Class D switching amplifier being configured as a switching power supply having a positive input, a negative input, and an output to output a switching waveform used to form the gamma correction signal and a gamma feedback signal coupled to the negative input, and a control circuit coupled to the Class D switching amplifier, wherein the control circuit is configured to control the Class D switching amplifier so as to modulate a duty cycle of the switching waveform. 
 
     
     
       2. The liquid crystal display with gamma application circuit of  claim 1  wherein the DAC comprises a resistor string coupled to the analog voltage power supply and having a plurality of tap points to provide external control access to the resistor string, and a multiplexor having a plurality of inputs coupled to the plurality of tap points and an output coupled to the buffering amplifier, wherein a discrete gamma application circuit is coupled to all or a select subset of the tap points. 
     
     
       3. The liquid crystal display with gamma application circuit of  claim 1  wherein a voltage range output from the switching amplifier is between 200 mV and a voltage of the analog voltage power supply minus 200 mV. 
     
     
       4. The liquid crystal display with gamma application circuit of  claim 1  wherein a voltage range output from the switching amplifier is between 100 mV and a voltage of the analog voltage power supply minus 100 mV. 
     
     
       5. The liquid crystal display with gamma application circuit of  claim 1  wherein a voltage range output from the switching amplifier is between 10 mV and a voltage of the analog voltage power supply minus 10 mV. 
     
     
       6. A liquid crystal display with gamma application circuit comprising:
 a liquid crystal display having at least one source driver chip provided with an input stage comprising a digital-to-analog converter (DAC) followed by a buffering amplifier; and 
 a gamma application circuit coupled to the source driver chip to supply the source driver chip with a gamma correction signal, wherein the gamma application circuit comprises a Class D switching amplifier with gamma feedback that is configured as a switching power supply with a positive input, a negative input, and an output to output a switching waveform comprising the gamma feedback coupled to the negative input, a discrete inductor coupled to the output of the Class D amplifier and configured to receive the switching waveform and output the gamma correction signal, a capacitor coupled to the inductor and a control circuit configured to control the Class D switching amplifier so as to modulate a duty cycle of the switching waveform; 
 wherein the Class D switching amplifier is coupled to an analog voltage power supply; 
 wherein a power efficiency of the Class D switching amplifier is equal to or greater than 80%; and 
 wherein a voltage range output from the Class D switching amplifier is between 100 mV and a voltage of the analog voltage power supply minus 100 mV. 
 
     
     
       7. The liquid crystal display with gamma application circuit of  claim 6  wherein the DAC comprises a resistor string coupled to the analog voltage power supply and having a plurality of tap points to provide external control access to the resistor string, and a multiplexor having a plurality of inputs coupled to the plurality of tap points and an output coupled to the buffering amplifier, wherein the gamma application circuit is coupled to all or a select subset of the tap points. 
     
     
       8. The liquid crystal display with gamma application circuit of  claim 6  wherein a voltage range output from the Class D switching amplifier is between 10 mV and a voltage of the analog voltage power supply minus 10 mV. 
     
     
       9. A method of driving a liquid crystal display having one or more source driver chips, the method comprising:
 a. using a Class D switching amplifier configured as a switching power supply with a positive input, a negative input, and an output, the Class D switching amplifier being separate from the display having the one or more source driver chips, the switching amplifier configured as a switching power supply to supply a switching waveform and having a gamma feedback coupled to the negative input; 
 b. electronically controlling the Class D switching amplifier to modulate a duty cycle of the switching waveform; 
 c. forming a gamma correction signal by filtering the switching waveform with a filter comprising a discrete inductor and a capacitor; 
 d. converting the gamma correction signal to an analog signal in an input stage of the one or more source driver chips; and 
 e. buffering and amplifying the analog signal for gamma correction in the one or more source driver chips.